In your WP-Admin on the left go to USERS > click the grey "ADD NEW" button. Note, you'll choose their role (subscriber, administrator, etc). Make sure to put the correct role so they only have access to what they need.
If you are using self-hosted wordpress you may follow -> Milosovic Bejdak & criticalsector's instruction. And for wordpress.com the instruction is same but the option would be "Invite New" This will be very complex for him, i think.